Improved process and system for vapor phase polymerization of olefin monomers

ABSTRACT

The present invention relates to a continuous vapor phase olefin polymerization process comprising polymerization of at least one olefin monomer in at least two serial polymerization reactors containing an agitated bed of forming polymer particles, wherein forming polymer particles are transferred from an upstream reactor to a downstream reactor, wherein the upstream reactor is a horizontal stirred reactor containing multiple reaction zones, each reaction zone having at least one inlet for a gaseous stream and optionally additionally an inlet for a liquid stream, wherein said process reduces the carry-over of undesired reactive gases from the upstream reactor to the downstream reactor. The present invention further relates to a system suitable for the present continuous vapor phase olefin polymerization process. The present invention further relates to the use of the present process and system for producing heterophasic polypropylene.

The present invention relates to a continuous vapor phase olefinpolymerization process comprising polymerization of at least one olefinmonomer in at least two serial polymerization reactors containing anagitated bed of forming polymer particles, wherein forming polymerparticles are transferred from an upstream reactor to a downstreamreactor, wherein the upstream reactor is a horizontal stirred reactorcontaining multiple reaction zones, each reaction zone having at leastone inlet for a gaseous stream and optionally additionally an inlet fora liquid stream, wherein said process reduces the carry-over ofundesired reactive gases from the upstream reactor to the downstreamreactor. The present invention further relates to a system suitable forthe present continuous vapor phase olefin polymerization process. Thepresent invention further relates to the use of the present process andsystem for producing heterophasic polypropylene.

The prior art previously described continuous olefin polymerizationprocesses comprising polymerization of at least one olefin monomer in atleast two serial vapor phase polymerization reactors containing anagitated bed of forming polymer particles, comprising a polymerparticles transfer step wherein forming polymer particles aretransferred from an upstream reactor to a downstream reactor.

U.S. Pat. No. 4,420,592 describes a process for the polymerization of anolefin in the gaseous phase in a multiplicity of steps in at least twoindependent polymerization zones connected to each other by a transferpassage, which comprises feeding an olefin and a catalyst into a firstpolymerization zone, polymerizing the olefin in the gaseous phase,intermittently or continuously withdrawing a gaseous stream containingthe resulting polymer from the first zone and feeding it into saidtransfer passage, introducing the withdrawn polymer-containing gaseousstream into a second polymerization zone through the transfer passage,feeding the olefin into the second zone, wherein an inert gas zone isprovided in the transfer passage and at least a part of the gascomponents of the gaseous stream containing the polymer is replaced byan inert gas. It is taught in U.S. Pat. No. 4,420,592 that bysubstituting an inert gas for at least a part of the polymer-containinggaseous stream in the transfer passage it is possible to select desiredpolymerization conditions for the second polymerization zone.

U.S. Pat. No. 6,069,212 describes a method and apparatus for continuousvapor phase polymerization of a polymerizable monomer or mixture thereofto produce normally solid polymer substances in two or more seriallydisposed vapor phase polymerization reactors, each containing aquench-cooled subfluidized particulate bed of polymerized monomers,which allows maintaining each reactor at independently selectedoperating conditions. The process of U.S. Pat. No. 6,069,212 includes:

-   -   (a) discharging a slug containing polymer particles and reactive        gases from an upstream reactor,    -   (b) collecting the polymer particles in a transfer chamber        having side walls which are vertical and/or inclined toward a        bottom discharge port at an angle of less than about 20° from        vertical, while maintaining the pressure therein at least 5 psi        (35 kPa) below the operating pressure of the upstream reactor,    -   (c) repeating steps (a) and (b) to detach a suitable mass of        polymer particles from the bed in the upstream reactor,    -   (d) displacing a substantial portion of the reactive gases from        the collected polymer particles and the transfer chamber with a        purge gas having a composition which is compatible with the        independently selected operating conditions in each reactor, and        different from the composition of reactive gases in the upstream        reactor,    -   (e) pressurizing the transfer chamber gas pressure to at least 1        psi (7 kPa), preferably at least 2 psi (14 kPa), above the        operating pressure of the downstream reactor to facilitate        transfer of the polymer particles from the transfer chamber into        the downstream reactor, and    -   (f) dumping the polymer particles from the transfer chamber into        the downstream reactor.

CN1887916 discloses continuous vapor phase polymerization of olefinhomopolymers in two serially disposed vapor phase polymerizationreactors separated by partition boards into several polymerizationareas.

A major drawback of the processes of U.S. Pat. Nos. 4,420,592 and6,069,212 is that the described substitution of at least a part of thepolymer-containing gaseous stream by an inert gas or an purge gas in thetransfer passage is not sufficient to eliminate the carry-over ofcertain components comprised in the gaseous stream containing thepolymer that is withdrawn from the first/upstream reaction zone orreactor to the second/downstream reaction zone or reactor.

It was an object of the present invention to provide a continuous olefinpolymerization process comprising polymerization of at least one olefinmonomer in at least two serial vapor phase polymerization reactorscontaining an agitated bed of forming polymer particles wherein thecarry-over of reactive gases from the upstream reactor to the downstreamreactor is further reduced.

The solution to the above problem is achieved by providing theembodiments as described herein below and as characterized in theclaims. This process is also presented in FIG. 2 which is furtherdescribed herein below.

Accordingly, the present invention provides a continuous vapor phaseolefin polymerization process comprising polymerization of at least oneolefin monomer in at least two serial polymerization reactors containingan agitated bed of forming polymer particles,

wherein forming polymer particles are transferred from an upstreamreactor to a downstream reactor,wherein the upstream reactor is a horizontal stirred reactor containingmultiple reaction zones, each reaction zone having at least one inletfor a gaseous stream and optionally additionally an inlet for a liquidstream and,wherein one reaction zone comprised in the upstream reactor vessel is apolymer discharge reaction zone from which forming polymer particles aredischarged and subsequently transported to the downstream reactor andwherein the ratio of the hydrogen concentration to the olefin monomerconcentration in the polymer discharge reaction zone([H₂]_(discharge zone)/[Olefin monomer]_(discharge zone)) is reducedcompared to the ratio of the hydrogen concentration to the olefinmonomer concentration in the preceding reactor zone([H₂]_(preceding zone)/[Olefin monomer]_(preceding zone)).

In other words, the present invention provides a continuous vapor phaseolefin polymerization process comprising polymerization of at least oneolefin monomer in at least two serial polymerization reactors containingan agitated bed of forming polymer particles,

wherein forming polymer particles are transferred from an upstreamreactor to a downstream reactor,

wherein the upstream reactor is a horizontal stirred reactor containingmultiple reaction zones, each reaction zone having at least one inletfor a gaseous stream and optionally additionally an inlet for a liquidstream and,

wherein one reaction zone comprised in the upstream reactor vessel is apolymer discharge reaction zone from which forming polymer particles aredischarged and subsequently transported to the downstream reactor andwherein

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{receding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$

is more than 1.

Preferably,

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$

is more than 1.1, more preferably more than 1.2, even more preferablymore than 1.3 and particularly preferably more than 1.4.

In the context of the present invention, it was surprisingly found thatby reducing the ratio of the hydrogen concentration to the olefinmonomer concentration in the polymer discharge reaction zone compared tothe ratio of the hydrogen concentration to the olefin monomerconcentration in the preceding reactor zone, the carry-over of hydrogenfrom the upstream reactor to the downstream reactor can be significantlyreduced, without inducing a significant negative impact on thecharacteristics of the polymer product produced by the process of thepresent invention.

Preferably, the ratio

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$

is 1.5-15, more preferably the ratio

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$

is 2-10 and most preferably the ratio

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}\mspace{14mu} {is}\mspace{14mu} 3\text{-}7.$

The prior art teaches that the reactive gases discharged with theforming polymer powder to the gas-solid separator/pressure transferchamber system can be removed by means of purging with a gas compatiblewith the conditions in the downstream reactor. However, it was foundthat the effect of the purging is not optimal, particularly whenproducing an impact PP copolymer. Without being bound to theory, it isbelieved that part of the reaction gases carried-over from the upstreamreactor to the downstream reactor is contained in the pores and/orinternal voids of the forming polymer particles. In order to minimizethe carry-over at least a part of these reaction gases comprised in theforming polymer particle pores must have a composition that iscompatible with the conditions in the downstream reactor before saidforming polymer particles are discharged into the downstream reactor. Itwas surprisingly found that this effect can be achieved using theprocess and the system of the present invention without having asignificant negative impact on the characteristics of the polymerparticles produced by the process of the present invention. Particularlyfor materials with a high melt index, such as a PP homopolymer producedin an upstream polymerization reactor used in a process for producingheterophasic polypropylene copolymer grades, the intraparticle porositymay be relatively high as the forming PP polymer particles areparticularly porous. The purging step as proposed in the prior art doesnot have a significant effect on the reactive gases contained in theintraparticle space, resulting in that these reactive gases arecarried-over into the downstream reactor. Particularly in a process forthe production of high impact PP copolymer grades, the amount ofhydrogen entering the downstream polymerization reactor via the formingpolymer powder transfer system may be higher than the hydrogen contentthat can be tolerated when producing a low melt flow index material.

Preferably, one or more liquid streams to one or more reaction zonesand/or one or more gaseous streams to one or more reaction zones arecontrolled to achieve the desired ratios of the hydrogen concentrationto the olefin monomer concentration in the reaction zones.

Preferably, the liquid stream to at least one reaction zone is a liquidreactor recycle stream optionally further comprising an inert diluent orthe olefin monomer, wherein said inert diluent is preferably one or moreselected from the group consisting of nitrogen, fuel gas, methane,ethane and propane and wherein said olefin monomer preferably isethylene and/or propylene. The olefin monomer in the liquid recyclestream is the olefin used for producing the polymer particles. In aprocess for producing polypropylene, accordingly, the olefin monomer inthe liquid recycle stream preferably is propylene, whereas in a processfor producing random PP copolymer the olefin monomer in the liquidrecycle stream preferably is a mixture of ethylene and propylene. Theeffect of using the monomer is that the discharge zone still contributesto the reaction time.

Preferably, the gaseous stream to at least one reaction zone comprises agaseous recycle stream optionally further comprising an inert diluent orthe olefin monomer, wherein said inert diluent is preferably one or moreselected from the group consisting of nitrogen, fuel gas, methane,ethane and propane and wherein said olefin monomer preferably isethylene and/or propylene. The olefin monomer in the gaseous recyclestream preferably is the olefin used for producing the polymerparticles. In a process for producing polypropylene, accordingly, theolefin monomer in the gaseous recycle stream preferably is propylene,whereas in a process for producing random PP copolymer the olefinmonomer in the gaseous recycle stream preferably is ethylene andpropylene.

Preferably, the liquid stream to the polymer discharge reaction zoneand/or the gaseous to the polymer discharge reaction zone is propylene.Preferably, the propylene is polymer grade propylene consisting of atleast 99.7 wt-% propylene. The upstream reactor used in the process ofthe present invention is a horizontal stirred reactor, preferably withsemi-plug flow behavior.

Preferably, forming polymer particles are transferred from the upstreamreactor to the downstream reactor in a forming polymer particlestransport step comprises in a repeating sequence the steps of:

(a) discharging at least one charge of forming polymer powder andreactive gases from the upstream reactor into a gas-solid separatorwherein the polymer powder is separated from the reactive gases;

(b) collecting the polymer powder separated in the gas-solid separatorin a pressure transfer chamber;

(c) increasing the pressure in the pressure transfer chamber with apressurizing gas to a pressure that is higher than the operatingpressure of the downstream reactor;

(d) discharging the polymer powder from the pressure transfer chamberinto the downstream reactor, and

(e) opening the valve between the gas-solid separator and the pressuretransfer chamber

wherein the pressure in the pressure transfer chamber before saiddischarging at least one charge of forming polymer powder and reactivegases from the upstream reactor into the gas-solid separator is 10-700kPaa, preferably 90-500 kPaa and most preferably 105-200 kPaa.

In the context of the present invention, it was surprisingly found thatby reducing the pressure in the pressure transfer chamber beforedischarging at least one charge of forming polymer powder and reactivegases from the upstream reactor into the gas-solid separator is 10-700kPaa, preferably at 90-500 kPaa and most preferably at 105-200 kPaa, thecarry-over of hydrogen from the upstream reactor to the downstreamreactor can be further reduced.

Accordingly, the pressure in the pressure transfer chamber beforedischarging at least one charge of forming polymer powder and reactivegases from the upstream reactor into the gas-solid separator is at least10 kPaa (kPa absolute pressure), preferably at least 90 kPaa, mostpreferably at least 105 kPaa and not more than 700 kPaa, preferably notmore than 500 kPaa and most preferably not more than 200 kPaa. Ingeneral, a more reduced pressure, such as a pressure that is below theambient pressure, leads to a reduction in the carry-over of undesiredcomponents comprised in the reactive gases of the upstream reactor tothe downstream reactor. However, it may be advantageous to select apressure that is slightly above the ambient pressure to reduce the riskof leakage of atmospheric gases into the gas-solid separator/pressuretransfer chamber system. The presence of certain atmospheric gases, suchas oxygen, in the reactors may disturb the polymerization reaction.

Accordingly, the continuous olefin polymerization process of the presentinvention preferably comprises a polymer particles transfer stepcomprising a repeating sequence of steps, wherein the last step in thesequence is followed again by the first step in the sequence, therebyachieving a continuous transfer of forming polymer particles from theupstream reactor via the forming polymer powder transfer system to thedownstream reactor.

The polymer particles transfer step in a first substep (a) may comprisethat at least one charge of forming polymer powder and reactive gasesfrom the upstream reactor is discharged into a gas-solid separator. Thisfirst step (a) preferably involves opening and subsequently closing avalve between the upstream reactor and the gas-solid separator.Gas-solid separators suitable in the process of the present inventionmay function as a cyclone and are well-known in the art. For instance,the gas-solid separator may have an elongates shape having for theforming polymer powder and reactive gases from the upstream reactor atthe side of the gas-solid separator, an outlet for the offgas,preferably towards the offgas gas compressor, at the top of thegas-solid separator and an outlet of the polymer powder towards thepressure transfer chamber at the bottom of the gas-solid separator. Step(a) of the present invention may involve discharging more than onecharge, for instance up to 8 charges, preferably up to 6 charges, morepreferably up to 4 charges of forming polymer powder and reactive gasesinto the gas-solid separator.

The polymer particles transfer step in a subsequent substep (b) maycomprise that the polymer powder separated in the gas-solid separator iscollected in a pressure transfer chamber. Said pressure transfer chamberis preferably situated underneath the gas-liquid separator so that thepolymer powder is collected in the pressure transfer chamber by gravity.

Preferably following substep (b) of the polymer particles transfer step,the gases in the pressure transfer chamber and gas-solid separator aredisplaced with a purge gas. The optional purging step with a purge gasdisplaces the reactive gases comprised in the gas-solidseparator/pressure transfer chamber system, particularly the reactivegases comprised in the interparticle space of the forming polymerpowder. Any gas that is compatible with the conditions in the downstreamreactor may be selected as purge gas. The purge gas may be selected fromthe group consisting of one or more selected from the group consistingof nitrogen, fuel gas, methane, ethane propane, ethylene and propylene,preferably propylene, most preferably polymer grade propylene consistingof at least 99.7 wt-% propylene.

The polymer particles transfer step in a subsequent substep (c) maycomprise that a valve between the gas-solid separator and the pressuretransfer chamber is closed. The closing of this valve allows that thepressure in the pressure transfer chamber is increased with apressurizing gas to a pressure that is higher than the operatingpressure of the downstream reactor. Any gas that is compatible with theconditions in the downstream reactor may be selected as pressurizinggas. The pressurizing gas may be selected from the group consisting ofone or more selected from the group consisting of nitrogen, fuel gas,methane, ethane propane, ethylene and propylene, preferably propylene,most preferably polymer grade propylene consisting of at least 99.7 wt-%propylene.

The polymer particles transfer step in a subsequent substep (d) maycomprise that the polymer powder is discharged from the pressuretransfer chamber into the downstream reactor. This discharging steppreferably comprises opening and subsequently closing a valve betweenthe pressure transfer chamber and the downstream reactor. The transportof the forming polymer particles from the pressure transfer chamber tothe downstream reactor may be facilitated by a pressure gradient betweenthe pressure transfer chamber and the subsequent reactor.

The polymer particles transfer step in a subsequent substep (e) maycomprise that the valve between the gas-solid separator and the pressuretransfer chamber is opened. The opening of this valve is required toallow equalizing of the pressure in the pressure transfer chamber andthe gas-solid separator. The collecting of forming polymer powder in thepressure transfer chamber may be started again as soon as the pressuredifference between the pressure transfer chamber and the gas-solidseparator allows this and the valve between the pressure transferchamber and the gas-solid separator is opened.

Preferably, the gas-solid separator is in open gas communication withthe inlet of an offgas gas compressor during steps (a)-(e). It wassurprisingly found that by selecting a process wherein the gas-solidseparator is in open gas communication with the inlet of an offgas gascompressor during steps (a)-(e), the carry-over of hydrogen from theupstream reactor to the downstream reactor can be further reduced. Thismeans that a similar reduction in hydrogen carry-over can be obtained ata less reduced pressure in the pressure transfer chamber beforedischarging the forming polymer powder from the pressure transferchamber into the downstream reactor. Yet, there is preferably a valvesituated between the gas-solid separator and the inlet of the offgas gascompressor that remains open during the repeating sequence of steps(a)-(e).

In the process of the present invention, the offgas gas compressor mustbe able to achieve lower pressures in the gas-solid separator/pressuretransfer chamber system when compared to the processes of the prior art.Preferably, the offgas gas compressor is a multistage offgas gascompressor. A multistage offgas gas compressor, for instance a two-stageoffgas gas compressor, is particularly suitable to achieve the requiredpressure according to the present invention.

Preferably, the gas stream from the gas-solid separator to the inlet ofthe offgas gas compressor is subjected to a second gas-solid separator,preferably a cyclone. Such an additional gas-solid separator may beparticularly advantageous to remove the remaining forming polymerparticles, particularly polymer fines. This additional gas-solidseparator reduces the risk that the optional solid polymer particlefilter that is situated in the offgas feed to the offgas gas compressoris blocked quickly with fines, requiring frequent filter changes.

Preferably, the polymer particles transfer step is performed in twoalternating repeating sequences. Said two alternating repeatingsequences is performed in a staggered mode so that the second repeatingsequence is started when the first repeating sequence is halfway. Suchan operation in two alternating repeating sequences allows for an easiercontinuous operation of the olefin polymerization process.

Preferably, each repeating sequence takes 60-600 seconds, preferably100-300 seconds, most preferably 120-240 seconds.

Preferably, the process of the present invention involves operating theupstream reactor at different process conditions than the downstreamreactor. The pressure is usually similar in both reactors. The mostprominent reaction condition that may be different in the upstreamreactor when compared to the downstream reactor is the ratio between thehydrogen concentration and the olefin monomer concentration.Particularly in a process for producing impact PP copolymer grades, theratio between the hydrogen concentration and the olefin monomerconcentration in the upstream reactor is much higher than the olefinmonomer concentration in the downstream reactor. It should be noted thatwhen “hydrogen concentration” is stated, this is the concentration ofhydrogen, in the gas phase. It should be noted that when “olefin monomerconcentration” or “olefin concentration” or “monomer concentration” isstated, this is the concentration of olefin monomer in the gas phase.The concentration of hydrogen as well as the concentration of olefin ismeasured using a conventional GC that is calibrated using standardmixtures and the unit is mol/m³.

The process conditions useful in the process of the present invention,also described herein as “polymerization conditions”, can be easilydetermined by the person skilled in the art; see e.g. Lieberman et al.(2006) Polypropylene. Kirk-Othmer Encyclopedia of Chemical Technology.Accordingly, the process conditions in the reactor preferably include atemperature (reactor temperature) of 50-90° C. and a pressure (reactorpressure) of 1500-3000 kPa gauge (15-30 barg). The hydrogen to olefinmol-ratio (H2/olefin ratio) is set such as to obtain the requiredpolymer, based on the kinetics of the used catalyst system. Anyconventional catalyst systems, for example, Ziegler-Natta or metallocenemay be used. Such techniques and catalysts are described, for example,in WO06/010414; Polypropylene and other Polyolefins, by Ser van der Ven,Studies in Polymer Science 7, Elsevier 1990; WO 06/010414, U.S. Pat.Nos. 4,399,054 and 4,472,524. The present invention is particularlyuseful in combination with catalysts that produce a polymer with arelatively high internal porosity, such as catalysts that produce low(bulk) density polymers, particularly low (bulk) density PP.

In a further aspect of the present invention, a system suitable for acontinuous vapor phase olefin polymerization process according to thepresent invention is provided: a system suitable for a continuous vaporphase olefin polymerization process, comprising at least two serialpolymerization reactors containing an agitated bed of forming polymerparticles and a means to transfer forming polymer particles from anupstream reactor to a downstream reactor, wherein the upstream reactoris a horizontal stirred reactor containing multiple reaction zones, eachreaction zone having at least one inlet for a gaseous stream andoptionally additionally an inlet for a liquid stream and wherein onereaction zone comprised in the upstream reactor is a polymer dischargereaction zone comprising an outlet for forming polymer particles to themeans to transfer the forming polymer particles to the downstreamreactor and a means to achieve that the ratio of the hydrogenconcentration to the olefin monomer concentration in the polymerdischarge reaction zone ([H₂]_(discharge zone)/[Olefinmonomer]_(discharge zone)) is reduced compared to the ratio of thehydrogen concentration to the olefin monomer concentration in thepreceding reactor zone[H₂]_(preceding zone)/[Olefinmonomer]_(preceding zone)). This system of the present invention and theprocess as performed in said system is inter alia presented in FIG. 2(FIG. 2) and FIG. 3 (FIG. 3), whereas a system according to the priorart is presented in FIG. 1. It should be noted that the dotted linesshown in FIG. 3 show imaginary boundaries between zones; no physicalbarriers are present in this specific embodiment. In an embodiment (notshown) the discharge zone may be subdivided into multiple (for example2) imaginary zones; in particular if the length of the so-calleddischarge zone comprises a large section of the reactor.

Accordingly, the present invention provides a system comprising at leasttwo serial polymerization reactors containing an agitated bed of formingpolymer particles and a means to transfer forming polymer particles froman upstream reactor to a downstream reactor, wherein the upstreamreactor is a horizontal stirred reactor containing multiple reactionzones, each reaction zone having at least one inlet for a gaseous streamand optionally additionally an inlet for a liquid stream and wherein onereaction zone comprised in the upstream reactor is a polymer dischargereaction zone comprising an outlet for forming polymer particles to themeans to transfer the forming polymer particles to the downstreamreactor and a means to achieve that the ratio of the hydrogenconcentration to the olefin monomer concentration in the polymerdischarge reaction zone ([H₂]_(discharge zone)/[Olefinmonomer]_(discharge zone)) is reduced compared to the ratio of thehydrogen concentration to the olefin monomer concentration in thepreceding reactor zone ([H₂]_(preceding zone)/[Olefinmonomer]_(preceding zone)) It should be noted that no physical zonesegmentation (viz. no barriers between the zones) is needed andpreferably no physical zone segmentation (viz. no barriers between thezones) is present. This has the effect that the risk of polymer powder(especially low molecular weight random PP) sticking or reactor blockingis reduced or eliminated compared to reactors having physical barriers.This process thus allows for a multipurpose use of the reactor withoutphysical barriers. In an embodiment, the discharge zone has a volumethat comprises between 20 and 50%, such as 20 to 40% preferably between20 and 25% of the total volume of the reactor, in other words between ⅕and ½ of the reactor volume, preferably between ⅕ and ¼ of the reactorvolume. In an embodiment, each zone is ¼ or ⅕ of the total reactorvolume and preferably in such an embodiment, the discharge zone is onezone, the remaining 3 or 4 zones (respectively) are the reactor zones.In an embodiment wherein the discharge zone contributes to 40 or 50%;this may be seen as ⅖ and 2/4 respectively and when each zone is ⅕ or ¼(respectively) this means the discharge zone is 2 zones out of a totalof 5, respectively 4 zones.

Preferably, the means to the ratio of the hydrogen concentration to theolefin monomer concentration in the polymer discharge reaction zonecomprises an inlet for a liquid reactor recycle stream optionallyfurther comprising an inert diluent or the olefin monomer.

Preferably, the means to the ratio of the hydrogen concentration to theolefin monomer concentration in the polymer discharge reaction zonecomprises an inlet for a gaseous recycle stream optionally furthercomprising an inert diluent or the olefin monomer.

In an embodiment, the upstream reactor comprises an internal barrier toprevent back-mixing of the forming polymer product from the polymerdischarge reaction zone to one or more other reaction zones comprised inthe upstream reactor; only in this embodiment there is a physicalbarrier Such an internal barrier has the effect of enhancing the plugflow behavior of the reactor, facilitating to maintain the differencebetween the reaction conditions in the polymer discharge reaction zonecompared with the preceding reaction zones. Accordingly, the presence ofan internal barrier in the reactor to prevent back-mixing of the formingpolymer product from the polymer discharge reaction zone to one or moreother reaction zones comprised in the upstream reactor allows achievinga much higher reduction of the ratio of the hydrogen concentration tothe olefin monomer concentration in the polymer discharge reaction zonecompared to the ratio of the hydrogen concentration to the olefinmonomer concentration in the preceding reactor zone. Preferably, theforming polymer particles are transferred from the upstream reactor tothe downstream reactor using a polymer particles transport meanscomprising a gas-solid separator, a pressure transfer chamber and anoffgas gas compressor, wherein the inlet of said offgas gas compressoris in continuous open gas communication with the gas-solid separator.

Preferably, the means to transfer forming polymer particles comprises agas-solid separator that is connected through a valve with a pressuretransfer chamber and a means to maintain the pressure in the pressuretransfer chamber at 10-700 kPaa, preferably at 90-500 kPaa and mostpreferably at 105-200 kPaa before discharging at least one charge offorming polymer powder and reactive gases from the upstream reactor intothe gas-solid separator.

Preferably, the means to maintain the pressure in the gas-solidseparator is an offgas gas compressor and wherein the inlet of saidoffgas gas compressor is in continuous open gas communication with thegas-solid separator.

Preferably, the means to transferforming polymer particles from anupstream reactor to a downstream reactor comprises two parallelgas-solid separators, two parallel pressure transfer chambers, and oneoffgas gas compressor.

Preferably, the offgas gas compressor is a multi-stage offgas gascompressor.

Preferably, the offgas from the gas-solid separator is subjected to asecond sequential gas-solid separator, wherein said second sequentialgas-solid separator preferably is a cyclone.

In a further aspect of the present invention, the use of the processaccording to present invention or the use of the system according topresent invention for producing heterophasic polypropylene is provided.

The following numerical references are used in FIGS. 1, 2 and 3:

-   1 Polymer powder discharge from upstream reactor (R1) to gas-solid    separator A-   2 Polymer powder discharge from upstream reactor (R1) to gas-solid    separator B-   3 Connection between gas-solid separator A and pressure transfer    chamber A-   4 Connection between gas-solid separator B and pressure transfer    chamber B-   5 Polymer powder discharge from pressure transfer chamber A to    downstream reactor (R2)-   6 Polymer powder discharge from pressure transfer chamber B to    downstream reactor (R2)-   7 Gas outlet from gas-solid separator A (containing finer polymer    particles)-   8 Gas outlet from gas-solid separator B (containing finer polymer    particles)-   9 Inlet stream (gas containing polymer) to the gas-solid separator    overhead filters-   10 Gas outlet from filter-   11 Compressed gas stream (preferably to be recycled back to    polymerization reactor, e.g. R1)-   12 Purge and pressurization lines for blowcase A-   13 Purge and pressurization lines for blowcase B-   14 Gas outlet from optional additional gas-solid separator-   15 Solids outlet from optional additional gas-solid separator    (preferably recycled back to one or more selected from the group    consisting of gas-solid separator A, gas-solid separator B, pressure    transfer chamber A, pressure transfer chamber B and upstream    reactor)-   16 Liquid (recycle) stream(s)-   17 Gas (recycle) stream(s)-   101 Upstream reactor (R1)-   101 a Preceding reactor zone(s)-   101 b Polymer discharge reaction zone-   201 Gas-solid separator A-   202 Pressure transfer chamber A-   203 Gas-solid separator B-   204 Pressure transfer chamber B-   205 Solid polymer particles filter (e.g. for retaining particles    bigger than 2 microns)-   206 Offgas gas compressor (206 a optional multi-stage offgas gas    compressor)-   207 Additional gas-solid separator-   301 Downstream reactor (R2)

It is noted that the invention relates to all possible combinations offeatures described herein, particularly features recited in the claims.

It is further noted that the term ‘comprising’ does not exclude thepresence of other elements. However, it is also to be understood that adescription on a product comprising certain components also discloses aproduct consisting of these components. Similarly, it is also to beunderstood that a description on a process comprising certain steps alsodiscloses a process consisting of these steps.

The present invention will now be more fully described by the followingnon-limiting Examples.

EXAMPLES

The development of a detailed model of the first polymerization reactorenabled the understanding of the thermodynamic and transport phenomenaoccurring in this system. This model was developed in Mobatec Modelleremploying correlations derived from literature (for the mass transportphenomena) and from in-house developed models (namely thermodynamicmodels, based on PC-SAFT) and it was fitted with plant data. Thesorption of the different gas components in the amorphous PP wasestimated by developing simplified empirical correlations able to mimicthe behaviour predicted by PC-SAFT.

In the examples, the results on the amount of hydrogen still present inthe powder at the inlet of the downstream reactor are expressed ashydrogen Take-over, which is the excess of hydrogen in the downstreamreactor. In other words, “hydrogen Take-over” is “hydrogen carry-overfrom pressure chamber to downstream reactor” minus “hydrogen consumptionin downstream reactor”. This means the hydrogen Take-over could also bea negative value and additional hydrogen would need to be fed to thedownstream reactor. This is a preferred situation with respect to thehydrogen/olefin control in downstream reactor.

The hydrogen Take-over in the base case is 712 g/h.

In the Examples the reactor is divided into four imaginary zones, eachbeing ¼ or 25% of the reactor volume. The discharge zone is the lastzone and comprises ¼ or 25% of the total reactor volume.

Example 1

No gas injection in polymer discharge reaction zone.

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}} = 5.4$

Hydrogen Take-over after applying this modification: 141 g/h

Reduction in hydrogen Take-over: 80%

Example 2

Manipulate the inlet of liquid and gas recycle streams is each zone ofthe first reactor.

Inject the minimum gas flow rate in polymer discharge reaction zone.

Injecting the minimum gas flow rate copes with the requirementsassociated with the nozzle operation.

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}} = {3{.2}}$

Hydrogen Take-over after applying this modification: 226 g/h

Reduction in hydrogen Take-over: 68%

Example 3

Inject pure gaseous propylene at the bottom of the polymer bed, insteadof recycled gas.

Inject pure gaseous propylene in the polymer discharge reaction zone.

The recycled gas is injected in all 3 preceding reaction zones. The flowof pure propylene is the minimum gas flow required to avoid the nozzleblockage.

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}} = 5.6$

Hydrogen Take-over after applying this modification: 140 g/h

Reduction in hydrogen Take-over: 80%

Example 4

Inject pure gaseous propylene at the bottom of the polymer bed (insteadof recycled gas) and inject pure liquid propylene as quench, at the topof the reactor (instead of the recycled liquid).

$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}} = {7{.6}}$

Hydrogen Take-over after applying this modification: −26 g/h

Reduction in hydrogen Take-over: 104%

1. A continuous vapor phase olefin polymerization process comprisingpolymerization of at least one olefin monomer in at least two serialvapor phase polymerization reactors containing an agitated bed offorming polymer particles, wherein forming polymer particles aretransferred from an upstream reactor to a downstream reactor, whereinthe upstream reactor is a horizontal stirred reactor containing multiplereaction zones, each reaction zone having at least one inlet for agaseous stream and optionally additionally an inlet for a liquid streamand, wherein one reaction zone comprised in the upstream reactor vesselis a polymer discharge reaction zone from which forming polymerparticles are discharged and subsequently transported to the downstreamreactor and wherein the ratio of the hydrogen concentration to theolefin monomer concentration in the polymer discharge reaction zone([H₂]_(discharge zone)/[Olefin monomer]_(discharge zone)) is reducedcompared to the ratio of the hydrogen concentration to the olefinmonomer concentration in the preceding reactor zone([H₂]_(receding zone)/[Olefin monomer]_(preceding zone)).
 2. The processaccording to claim 1, wherein the ratio$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$is 1.5-15.
 3. The process according to claim 1, wherein one or moreliquid streams to one or more reaction zones and/or one or more gaseousstreams to one or more reaction zones are controlled to achieve thedesired ratios of the hydrogen concentration to the olefin monomerconcentration in the reaction zones.
 4. The process according to claim3, wherein the liquid stream to at least one reaction zone is a liquidreactor recycle stream further comprising an inert diluent or the olefinmonomer.
 5. The process according to claim 3, wherein the gaseous streamto at least one reaction zone comprises a gaseous recycle stream furthercomprising an inert diluent or the olefin monomer.
 6. The processaccording to claim 1, wherein the liquid stream to the polymer dischargereaction zone and/or the gaseous to the polymer discharge reaction zoneis propylene.
 7. The process according to claim 3, wherein the propyleneis polymer grade propylene consisting of at least 99.7 wt-% propylene.8. The process according to claim 1, wherein the upstream reactorcomprises an internal barrier to prevent back-mixing of the formingpolymer product from the polymer discharge reaction zone to one or moreother reaction zones comprised in the upstream reactor.
 9. The processaccording to claim 1, wherein forming polymer particles are transferredfrom the upstream reactor to the downstream reactor in a forming polymerparticles transport step comprises in a repeating sequence the steps of:(a) discharging at least one charge of forming polymer powder andreactive gases from the upstream reactor into a gas-solid separatorwherein the polymer powder is separated from the reactive gases; (b)collecting the polymer powder separated in the gas-solid separator in apressure transfer chamber; (c) increasing the pressure in the pressuretransfer chamber with a pressurizing gas to a pressure that is higherthan the operating pressure of the downstream reactor; and (d)discharging the polymer powder from the pressure transfer chamber intothe downstream reactor, and (e) opening the valve between the gas-solidseparator and the pressure transfer chamber, wherein the pressure in thepressure transfer chamber before said discharging at least one charge offorming polymer powder and reactive gases from the upstream reactor intothe gas-solid separator is 10-700 kPaa.
 10. A system suitable for acontinuous vapor phase olefin polymerization process according to claim1, comprising at least two serial vapor phase polymerization reactorscontaining an agitated bed of forming polymer particles and a means totransfer forming polymer particles from an upstream reactor to adownstream reactor, wherein the upstream reactor is a horizontal stirredreactor containing multiple reaction zones, each reaction zone having atleast one inlet for a gaseous stream said reactor comprising multipleinlets for a gaseous stream to set different hydrogen to olefin ratiosand optionally additionally an inlet for a liquid stream and wherein onereaction zone comprised in the upstream reactor is a polymer dischargereaction zone comprising an outlet for forming polymer particles to themeans to transfer the forming polymer particles to the downstreamreactor.
 11. The system according to claim 10, wherein the means toreduce the ratio of the hydrogen concentration to the olefin monomerconcentration in the polymer discharge reaction zone comprises an inletfor a liquid reactor recycle stream optionally further comprising aninert diluent or the olefin monomer.
 12. The system according to claim10, wherein the means to reduce the ratio of the hydrogen concentrationto the olefin monomer concentration in the polymer discharge reactionzone comprises an inlet for a gaseous recycle stream optionally furthercomprising an inert diluent or the olefin monomer.
 13. The systemaccording to claim 10, wherein the upstream reactor comprises aninternal barrier to prevent back-mixing of the forming polymer productfrom the polymer discharge reaction zone to one or more other reactionzones comprised in the upstream reactor.
 14. The system according toclaim 10, wherein the forming polymer particles are transferred from theupstream reactor to the downstream reactor using a polymer particlestransport means comprising a gas-solid separator, a pressure transferchamber and an offgas gas compressor, wherein the inlet of said offgasgas compressor is in continuous open gas communication with thegas-solid separator.
 15. (canceled)
 16. The process according to claim1, wherein the ratio$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$is 2-10 wherein one or more liquid streams to one or more reaction zonesand/or one or more gaseous streams to one or more reaction zones arecontrolled to achieve the desired ratios of the hydrogen concentrationto the olefin monomer concentration in the reaction zones, wherein theliquid stream to at least one reaction zone is a liquid reactor recyclestream further comprising an inert diluent, wherein said inert diluentis one or more selected from the group consisting of nitrogen, fuel gas,methane, ethane and propane, wherein the gaseous stream to at least onereaction zone comprises a gaseous recycle stream further comprising aninert diluent, wherein said inert diluent is one or more selected fromthe group consisting of nitrogen, fuel gas, methane, ethane and propane,wherein the liquid stream to the polymer discharge reaction zone and/orthe gaseous to the polymer discharge reaction zone is propylene, whereinthe upstream reactor comprises an internal barrier to preventback-mixing of the forming polymer product from the polymer dischargereaction zone to one or more other reaction zones comprised in theupstream reactor,
 17. The process according to claim 16, wherein formingpolymer particles are transferred from the upstream reactor to thedownstream reactor in a forming polymer particles transport stepcomprises in a repeating sequence the steps of: (a) discharging at leastone charge of forming polymer powder and reactive gases from theupstream reactor into a gas-solid separator wherein the polymer powderis separated from the reactive gases; (b) collecting the polymer powderseparated in the gas-solid separator in a pressure transfer chamber; (c)increasing the pressure in the pressure transfer chamber with apressurizing gas to a pressure that is higher than the operatingpressure of the downstream reactor; and (d) discharging the polymerpowder from the pressure transfer chamber into the downstream reactor,and (e) opening the valve between the gas-solid separator and thepressure transfer chamber, wherein the pressure in the pressure transferchamber before said discharging at least one charge of forming polymerpowder and reactive gases from the upstream reactor into the gas-solidseparator is 10-700 kPaa, preferably 90-500 kPaa and most preferably105-200 kPaa.
 18. The process according to claim 16, wherein the ratio$\frac{\lbrack H_{2} \rbrack_{{preceding}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{preceding}\; {zone}}}{\lbrack H_{2} \rbrack_{{discharge}\; {zone}}/\lbrack {{Olefin}\mspace{14mu} {monomer}} \rbrack_{{discharge}\; {zone}}}$is 3-7.
 19. The process according to claim 16, wherein the propylene ispolymer grade propylene consisting of at least 99.7 wt-% propylene.